But here is the difference between 1900, 1960 and 2023. The technological advancement and correspondingly the necessary know how is tremendous.
But in the typical 6-12 months the training you can give is pretty much the same over all these periods. That is basic fitness, basic operation of the equipment, basic tactics and some shooting.
In 1900 that covered most of the necessary abilities, as most soldiers were infantrists fighting with rifles and swords in trenches. In 1960 that covered half the necessary abilities, because the grunts were embedded in mechanized units and got support from things like artillery and helicopters that they would need to call in and guide.
In 2023 an experienced squad with a competent drone operator, using a simple commercial drone will just call in the modern artillery system that is standing 40km away on your grunts position and they never knew they were spotted, leave alone in danger until the split second before impact.
Having millions of reservists called in, that received some training 10 years ago and can shoot a rifle is worth much much less nowadays. What we need is a properly funded and trained professional army, that is attractive as an employer to get competent people to join.